Evoked EMG in juvenile ocular myasthenia gravis

We studied juvenile ocular myasthenia gravis (MG) with special reference to evoked EMG in orbicularis oculi muscle. The subjects consisted of 5 cases aged 1-4 years and 3 cases over 10 years. Young children were examined during drug-induced sleep. Stimulation was delivered to preauricular facial nerve, and M waves were recorded from ipsilateral orbicularis oculi muscle. Examinations were also carried out in thenar or hypothenar muscles by stimulating the median or ulnar nerves respectively. On Harvey-Masland test, 6 out of 8 cases showed waning of more than 10%. On M-wave recovery cycle, 3 out of 6 cases examined showed decrement during stimulation interval of 100 msec to 500 msec. In 2 of these, the peak of recovery cycle curve rose over 100% during stimulation interval of 30 to 60 msec on a usual dose treatment of anti-cholinesterase, while on prednisolone treatment this peak did not increase over 100% and the pattern of recovery cycle changed to almost normal. On post-tetanic cycle study, only 1 out of 4 cases examined showed post-tetanic facilitation and exhaustion. The response of the belly-tendon in the hand showed no abnormality. Although there still remains technical difficulty in the examination of evoked EMG in orbicularis oculi muscle, this examination is very useful in diagnosis, evaluation and understanding of pathophysiology of MG.